Boron phosphide (BP) is a lightweight compound with significant hardness, and is thermally stable and extremely corrosion-resistant against strong reagents. BP is ill-understood in terms of its unique properties. For example, BP is lightweight but extremely hard, and resistant to strong corrosive reagents, however, these properties have not yet been exploited in any potential applications. In addition, there is a growing interest in recent years in using so-called “ceramic-type” materials, such as fine boron carbide or silicon carbide powder, as environmentally benign fuels in pyrotechnic formulations for their long lasting and controllable burning characteristics.
Long regarded as a promising III-V compound semiconductor, boron phosphide (BP) is characterized by a wide indirect band gap of 2.0 eV along with high electron and hole mobility at elevated temperatures. Both n-type (with an excess of phosphorus) and p-type (with an excess of boron) BP semiconductors are being studied, and a number of BP-based optoclectronic and thermoelectric devices, such as Schottky harriers, metal-insulator-semiconductor (MIS) junctions, and p-n electroluminescent junctions, have since been fabricated. BP microcrystals are also being considered as potential heat-sink substrates for semiconductor devices owing to their high thermal conductivity comparable to that of boron nitride. Their potential application in solid-state neutron detectors, due to the large thermal neutron capture cross-section of the boron-10 isotope, has been reported. As a refractory material with a high refractive index of 3.0 at around 0.63 μm in the visible spectrum, its use in high temperature luminescent devices has been explored in the past as well.
Finely powdered boron carbide was recently proposed and demonstrated as an environmentally benign alternative to the toxic chemicals in a variety of pyrotechnic compositions including those for smoke production and time delay fuzes (Shaw, Diviacchi, et al. 2015; Shaw, Poret, et al. 2015). Microcrystalline BP is a potentially useful fuel for pyrotechnic compositions, especially those intended for smoke production, because of its high phosphorus content (74 percent by weight). Elemental phosphorus has been used for many years in high-performance smoke munitions. The unique properties of BP, along with its potential as an environmentally benign fuel in pyrotechnic formulations, however, have not yet been exploited because of the lack of a safe and economically feasible process for its large-scale production.
Synthesis of boron phosphide typically requires use of toxic components or complex processes that are unsafe and expensive. The first syntheses of BP from boron halides (BBr3 or BI3) and phosphine (PH3) or white phosphorus (P4) were reported in 1891. Since then, many other precursor chemicals, such as elemental boron, triethylborane (B(C2H5)3), diborane (B2H6) for boron and white phosphorus (P4), phosphorus halides (PCl3 or PCl5), aluminum phosphide (AlP), zinc phosphide (Zn3P2) for phosphorus, etc., have been used as source materials for BP synthesis. Typically, a precursor mixture is heated in a sealed tube to a high temperature or dropped directly into a superheated reaction zone in excess of 1000° C. The approach has been adopted reasonably well in the laboratory as a chemical vapor deposition (CVD) process to grow BP epitaxial layers on various supporting substrates, but the use of highly toxic and flammable reagents at high temperatures presents a significant challenge for any of those processes to be implemented safely and economically in a large-scale operation. A so-called solvothermal method was recently introduced to synthesize BP nanocrystals from boron powder and phosphorus trichloride in a sealed autoclave with benzene as the solvent. The synthesis was performed at a relatively lower temperature of about 350° C., in which the nascent phosphorus precursor was actually produced by reduction of the trichloride with metallic lithium or sodium.
Most recently, V. A. Mukhanov et al., reported a simplified process for “Self-Propagating High-Temperature Synthesis of Boron Phosphide”. This process employs a self-propagating high-temperature synthesis (SHS), to synthesize BP microcrystal particles from a highly consolidated pellet made of readily available boron phosphate (BPO4) and magnesium powders. The reaction, in which boron phosphate functions as an oxidizer for the oxidization of magnesium to magnesium oxide (MgO) while it is reduced to BP, is highly exothermic. Mukhanov described mixing fine boron phosphate and magnesium powders at a 1.0/4.1 molar ratio, and pressing the mixture at an extraordinarily high pressure of 0.6 GPa into a pellet 40 mm in diameter and 20 mm in length. The very high pressure specified for the pressing step corresponds to the application of 169,520 lbs-force (considering the diameter of 40 mm). The pellet was then heated with the flame of a gas burner to initiate the reaction, but the product of the reaction, however, was found to contain a significant amount (up to 30%) of the impurity boron subphosphide (B12P2). It was suggested that the impurity arose from immediate decomposition of the as-synthesized BP microcrystals due to an unexpected high reaction temperature of over 1227° C. The addition of sodium chloride into the mixture as a chemically inert diluent was shown to be necessary to reduce the reaction temperature and decrease the level of impurity. However, the incorporation of diluents can make SHS synthesis pellets difficult to ignite. Additionally, it was reported that the incorporation of sodium chloride can cause incomplete combustion of the synthesis pellets, resulting in decreased yields of BP. Mukhanov reported a yield of about 35% with respect to the theoretical maximum yield of BP, considering the reaction stoichiometry. With respect to the total mass of the starting materials, including the sodium chloride diluent, the reported yield of BP was even lower, less than 4%.
Thus, a need exists for a simplified process for producing boron phosphide in higher yields and with reduced amounts of impurities.